Hellschreiber is a morse-like graphical transmission of digital data reminiscent of fax over wireless communication media designed in the 1930s and used during the second world war by the German military. Now used by the ham radio community, we consider how hellschreiber is emitted using a Raspberry Pi GPIO pin configured as radiofrequency emitter, an approach made popular by rpitx and generalized to GNU Radio with our gr-rpitx OOT sink block (https://github.com/jmfriedt/gr-rpitx). The transmitted signal is recorded by a RTL-SDR DVB-T receiver and decoded using gr-hellschreiber (https://github.com/tlavarenne/gr-hellschreiber).
Preparing this presentation was an opportunity to become familiar with GNU Radio 3.9 and the presentation concludes with some of the challenges (and solutions) met when porting gr-acars
(https://sourceforge.net/projects/gr-acars/) to this new version of the signal processing framework.

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